1. Field of the Invention
The. present. invention relates to an ink (printing liquid) useful for writing utensils such as aqueous ball point pens, fountain pens, and felt-tip pens; on-demand type ink-jet printers of a thermal jet type, a piezo type, and the like; an ink set; an ink cartridge; a printing unit; an image printing apparatus; an ink-jet printing method employing the ink; and a coloring material for the ink.
2. Related Background Art
Dyes are used as the coloring material for inks for high color developability and high stability. However, the dyes are not satisfactory in water-resistance and light-fastness of the formed images, disadvantageously in spite of having several advantages, and therefore more improvement is required. In recent years, for offsetting the disadvantages, various inks are disclosed which employ a water-insoluble coloring material such as organic pigments and carbon black in place of the dyes.
However, even with the coloring material like an organic pigment or carbon black, the formed images are not satisfactory in rub resistance (for example, by rubbing with a finger, or overwriting with a felt-tipped pen). To solve this problem, use of a microcapsule is disclosed.
Japanese Patent Application Laid-Open No 1-170672 discloses an ink employing a capsuled coloring material prepared by dissolving or dispersing a coloring material in a water-insoluble solvent, dispersing and emulsifying the solution with a surfactant, and encapsulating the coloring material. Japanese Patent Application Laid-Open No. 62-95366 discloses an ink enclosing a dye ink in polymer particles. Japanese Patent Application Laid-Open No. 9-151342 discloses an aqueous liquid dispersion for printing liquid containing anionic microencapsulated pigment; the capsulated pigment containing the pigment at a content ranging from 35% by weight to 80% by weight.
The inventors of the present invention investigated comprehensively inks containing an organic pigment or carbon black microencapsulated. As the results, it was found that the ink containing the microencapsulated organic pigment gave insufficient color chroma, and was not able to provide an image having enough optical density, although such an ink has improved rub resistance with the image stability, such as light-fastness, maintained. Therefore, the insufficient color chroma or the optical density should be improved for a higher quality of image.
The inventors of the present invention confirmed that the above disadvantages are caused by covering of the organic pigment or carbon black with an organic polymer or the like. More specifically, the disadvantages are caused by the following two reasons: (1) The covering of the organic pigment or the carbon black with an organic polymer impairs color developability of the pigment, and (2) a residual matter remaining after the covering treatment lowers the surface tension of the ink to facilitate penetration of the organic pigment or carbon black of the ink into the interior of the printing medium.
The above two disadvantages can be overcome by lowering the degree of covering with the organic polymer. However, simple lowering of the covering degree may cause another problem that the organic pigment or carbon black are not sufficiently and finely dispersed in the ink medium. Because of the above problem, this method is not practical although the rub resistance can be improved with the image stability maintained without drop of the OD.
Further, for example, in color image printing with an ink set of inks of yellow, magenta, cyan, and so forth containing the microencapsulated organic pigment or carbon black, the color may run at the boundary between different colors, or the color inks may be mixed nonuniformly on the printing medium to lower the quality of the printed image. (This phenomenon is hereinafter called xe2x80x9cbleedingxe2x80x9d.)
To prevent such bleeding, Japanese Patent Application Laid-Open No. 55-65269, etc. disclose addition of a surfactant to the ink to accelerate penetration of the ink into the printing medium, and Japanese Patent Application Laid-Open No. 55-66976 discloses use of a volatile solvent as the main constituent of the ink liquid medium. These techniques also may cause decrease of image density or make the ink ejection unstable.
Accordingly, the inventors of the present invention recognized the necessity of development of the ink which contains an organic pigment or carbon black having improved color developability in single use or in combination with another color ink, and which causes less variation of image quality independently of the kind of the printing medium.
An object of the present invention is to provide an ink which lessens the influence of the printing medium on the printed image quality, gives high-quality images steadily, and is stable by itself.
Another object of the present invention is to provide an ink which forms high-quality images steadily independent of the printing medium.
Another object of the present invention is to provide an ink set which can prevent bleeding, by use of the aforementioned aqueous ink.
A further object of the present invention is to provide an image formation apparatus, an image forming method, and devices including an ink cartridge and a printing unit for the apparatus for steady formation of high-quality images with less influence of the printing medium on the image quality.
A still further object of the present invention is to provide a coloring material which improves the color chroma and density of the printed image and is suitable for ink-jet printing.
According to one aspect of the present invention, there is provided an aqueous ink comprising a water insoluble coloring material encapsulated with an organic polymer, wherein the coloring material encapsulated with the organic polymer contains the organic polymer in an amount of from 1% to 20% by weight to the aforementioned water insoluble coloring material.
According to another aspect of the present invention, there is provided an aqueous ink comprising a coloring material encapsulated with an organic polymer, wherein a part of the coloring material""s surface is substantially exposed.
The ink of still another embodiment of the present invention is an aqueous ink containing a capsuled coloring material constituted of a self-dispersible organic pigment or a self-dispersible carbon black as a coloring material, and an organic polymer covering the coloring material; the capsuled. coloring material containing the organic polymer at a content ranging from 1% to 20% by weight to said coloring material.
The ink of still another embodiment of the present invention is an aqueous ink containing a capsuled coloring material constituted of a self-dispersible organic pigment or a self-dispersible carbon black as a coloring material, and an organic polymer covering the coloring material; the coloring material being partly uncovered such as to be substantially exposed.
With the ink of the above embodiments excellent images can be formed with high rub resistance and high felt-pen resistance and with sufficient image density and color chroma.
The ink of still another embodiment of the present invention is an ink containing a capsuled coloring material constituted of a self-dispersible organic pigment or a self-dispersible carbon black as a coloring material, and an organic polymer covering the coloring material; the capsuled coloring material having an infrared absorption spectrum with a peak inherent to a surface functional group of the self-dispersible organic pigment or the self-dispersible carbon black.
The ink of still another embodiment of the present invention is an ink containing a capsuled coloring material constituted of a self-dispersible organic pigment or a self-dispersible carbon black as a coloring material, and an organic polymer covering the coloring material; the capsuled coloring material not giving a new absorption peak in infrared spectrum after removal of the resin.
With the ink of the above embodiments excellent images can be steadily formed with high rub resistance and high felt-pen resistance and with sufficient image density and color chroma.
To the above embodiments of the inks, there may be added effectively at least one salt selected from the group consisting of (M2)2SO4, CH3COO(M2), Phxe2x80x94COO(M2), (M2)NO3, (M2)Cl, (M2)Br, (M2)I, (M2)2SO3, and (M2)2CO3, where M2 is an alkali metal, ammonium, or an organic ammonium, and Ph is a phenyl group. The addition of the above salt can accelerate the separation of the coloring material (solid) and the aqueous medium (liquid) after adhesion of the ink onto a printing medium, thereby improving further the image density and color chroma.
The ink set for color printing of an embodiment of the present invention comprises inks of a black ink, a yellow ink, a magenta ink, and a cyan ink, wherein at least one of the inks is the ink described above.
With the ink set of the above embodiments excellent image can be steadily formed with high rub resistance and high felt-pen resistance and with sufficient image density and color chroma. To the ink of the above ink set, there may be added effectively at least one salt selected from the group consisting of (M2)2SO4, CH3COO(M2), Phxe2x80x94COO(M2), (M2)NO3, (M2)Cl, (M2)Br, (M2)I, (M2)2SO3, and (M2)2CO3, where M2 is an alkali metal, ammonium, or an organic ammonium, and Ph is a phenyl group. The addition of the above salt will accelerate the separation of the coloring material (solid) and the aqueous medium (liquid) after adhesion of the ink onto a printing medium, thereby improving further the image density and color chroma, and retarding the bleeding.
The ink cartridge of an embodiment of the present invention comprises an ink tank holding any of the above-mentioned inks.
The printing unit of an embodiment of the present invention has a holder for holding any of the above-mentioned inks, and a head for ejecting the ink.
The ink-jet printing method of an embodiment of the present invention comprises a step of flying any of the above-mentioned inks toward a printing medium surface and attaching the ink onto the printing medium surface.
The capsuled coloring material of an embodiment of the present invention is constituted of a self-dispersible organic pigment or a self-dispersible carbon black as a coloring material, and an organic polymer covering the coloring material; the capsuled coloring material having an infrared absorption spectrum having a peak inherent to a surface functional group of the self-dispersible organic pigment or the self-dispersible carbon black.
The capsuled coloring material of another embodiment of the present invention is constituted of a self-dispersible organic pigment or a self-dispersible carbon black as a coloring material, and an organic polymer covering the coloring material; the capsuled coloring material not giving a new absorption peak in infrared spectrum after removal of the covering of organic polymer.